Hi everyone. I'm thinking about trying my hand at making a raised bed this weekend. I got a book that has simple instructions on how to make one but I still have a few questions. Do I have to put a mat down or some type of tarp so that the new soil I buy doesn't interact with the soil that's in the yard which isn't all that great? Also I've read beds show be at least 12 inches deep. If I put a barrier on the ground under it is 12 inches enough for roots to grow in? I'm thinking of putting a few veggies in it, probably a squash plant and a tomatoe and the rest would be hummingbird attracting plants like Salvia and Foxglove.